Underactuated attitude stabilization for space tethered towing using constrained tension

During space tethered towing, the target and the space platform constitute a dumbbell-like tethered system, which only relies on the limited platform thrusts and tether tension to suppress the tether libration. An effective attitude stabilization strategy using bounded tension is proposed for such an underactuated system subject to input constraints. The dynamic model governing the attitude of the system is established. By solving the in-plane equilibrium equation numerically, the theoretical in-plane attitude commands are obtained, which are then optimized using Gauss pseudospectral method. The actual in-plane commands are thus yielded. An underactuated tension controller is designed based on hierarchical sliding mode theory, and an anti-windup auxiliary module is embedded to mitigate the tension saturation. Simulation results show that under the positive and constrained tension control, the tether can be deployed and retrieved smoothly by the platform, making the in-plane angle and tether length track the actual commands appreciably. The proposed strategy is also robust to the target swing and sensor errors.